1. Field of the Invention
This invention relates to a plastic packaged semiconductor device, and more particularly to a semiconductor device having bonding wires which are prevented from coming into contact with each other in a plastic sealing step.
2. Description of the Related Art
A plastic packaged semiconductor device is manufactured by mounting a pellet having semiconductor elements thereon on the bed of a lead frame and effecting a wire bonding step and a plastic packaging step by use of the transfer mold method. The lead frame may vary in construction according to the form of the package, the number of pins and the like, but is generally formed to have a plurality of inner lead portions which are formed on the surrounding portion of the bed acting as a support of the pellet to extend radially away from the bed and the bed is supported by tie bars. The bed is formed in a square or rectangular form which is similar to the form of the pellet and free ends of the inner lead portions are arranged on the surrounding portion thereof. The free ends of the inner lead portions and the bonding pad on the pellet are electrically connected to each other via bonding wires. The pellet, bed, inner lead portions and bonding wires are sealed in a plastic package. Outer lead portions which are connected to the inner lead portions are derived out to the exterior of the package.
In the semiconductor industry, it is desired to enhance the integration density, attain multifunction and lower the cost. In order to enhance the integration density and attain multifunction, the number of pins tends to increase, and at the same time, in order to lower the cost, the pellet size tends to be made smaller. As a result, a large number of bonding pads are formed on a small-sized pellet, and as a matter of course, a pitch between the bonding pads becomes smaller. At present, the pitch is formed to be approx. 200 .mu.m, but it is found that the following phenomenon will occur if it is formed to be less than 160 .mu.m. That is, if the plastic sealing step is effected under this condition, the bonding wire is forcedly flowed by gel plastic injected into a cavity mold so as to be made in contact with an adjacent bonding wire or the bonding pad. In particular, the bonding wire which is connected near the corner portion of the bed in position far apart from the injection port for the gel plastic is arranged with an acute angle with respect to the direction of arrangement of the bonding pads (the side of the bed) so that a distance to an adjacent bonding wire therefrom becomes small and it may be easily set into contact with the adjacent bonding wire. Further, at or around this portion, the injected gel plastic is being solidified and the flowability thereof is lowered than at a portion near the injection port so that a pressure applied to the bonding wire will become larger than that at the portion near the injection port, causing the bonding wire to be easily flowed.